This relates to decontamination and/or decommissioning of radioactively contaminated and/or corroded surfaces.
In the nuclear power industry, radioactive decontamination techniques of stainless steel components, other iron-based steels and alloys, metal surfaces, and other structural materials e.g., concrete, tools, etc. have been unsatisfactory for many applications due to ineffective scale removal, target specificity i.e. damage to the metal substrate, or waste handling problems. Chemical decontamination is achieved by the use of solvents to dissolve contaminated films or scale from the steel or metal substrates. Oxide scales are formed on stainless steels, iron-based alloys, and other non-ferrous surfaces in water systems at low and high temperatures and pressures. The dissolution of oxide scales can be achieved by injecting, circulating, and draining the chemical solvents from large equipment e.g. tanks, interior surfaces of pipes, coolant pipes and steam generators, and other facility components e.g., valves, tools. Decontamination of nuclear facilities is necessary to reduce radiation fields during daily operation, to facilitate eventual equipment handling and repairs, and for decommissioning and release of components. Currently, there are many available chemical techniques that can dissolve scales or films formed on ferrous metals, each with associated limitations. In order to develop more efficient chemical decontamination solvents, it is important to understand the formation of oxide scales. For boiling water reactors (BWRs) and pressurized water reactors (PWRs) there is a good understanding of oxide scale and/or film formation.
In general, there are two types of oxide scales formed in pipe interiors from commercial water-cooled reactors. The scale material serves as a trap for contaminants flowing in the coolant system. The exact composition of the scales is dependent on the type of commercial reactor (see Table 1) and coolant system chemistry (which may vary significantly from site to site). Typically two layers form. The inner layer is formed by corrosion of the metallic or alloy substrate and an outer layer, which, in general, is not strongly adhered to the substrate, is formed typically by a combination of corrosion, precipitation, and deposition of crud from the coolant.
This invention relates generally to the dissolution and treatment of minerals, oxides, scales typically found in both the BWRs and PWRs systems and other industrial facilities.
Contaminants in coolant systems are located in horizontal pipes, valves, pumps, heat exchanger, etc. The contaminants originate from activation and migration of dissolved stainless steel components (Table 2) or in some cases from defects in the fuel that permit the migration of fission products and actinides.
Various chemicals are used to decontaminate surfaces including organic acids, complexants, and mineral acids, see for instance Horwitz et al. U.S. Pat. No. 5,078,894 issued January, 1992, U.S. Pat. No. 5,332,531 issued Jul. 26, 1994, and U.S. Pat. No. 5,587,142 issued December, 1996 and the disclosure of which are incorporated herein by reference. The Waller et al. U.S. Pat. No 4,810,405 issued March, 1989 is also incorporated by reference. Many mineral acids (e.g., HF, HNO3, H2SO4) are used in decontamination solvents to dissolve oxide scales. Strong acids will dissolve the oxide scales; however they will also dissolve the metallic substrate. Typical dissolution rates of unreacted metal in mineral acids is significantly higher than the dissolution rate of oxidized metal. The dissolution of the metal substrate will deplete the acid effectiveness toward the oxide scale dissolution, increase waste volume, and compromise the structural integrity. Weak acids such as the organic acids (e.g., citric acid, oxalic acid, EDTA) are also added into decontamination solvent with the dual purpose of dissolving and complexing the dissolved metal oxide components. Some decontamination agents work primarily by dissolving the unreacted metal surface and uplifting the underlying grains and are not effective dissolution agents. This type of decontamination is not preferred in many decontamination scenarios since solids can accumulate in any dead leg or in elbows and lead to radioactive hotspots.
Various reviews have evaluated the need for decontamination and decommissioning (DandD) within DOE and surveyed DandD processes suitable for DOE applications. In general, decontamination of equipment prior to decommissioning does not require the protection of the base metal and thus may utilize the more aggressive decontamination agents; however, the acid treatment creates large volumes of waste that requires disposal. The chemical HEDPA is unique in that it can provide both protection to the base metal (important for continued operation of equipment) and large decontamination factors required for decommissioning.
The decontamination processes evaluated in this report is based on HEDPA (1-hydroxyethane-1,1-diphosphonic acid) 
and its equivalences for this purposes, as hereinafter set forth. An extensive review of HEDPA and compounds of the diphosphonic moeity, inorganic acids, and carboxylic acids was completed by Chiarizia and Horwitz and pubished as xe2x80x9cNew Formulations for Iron oxides Dissolutions, xe2x80x9cHydrometallurgy, 27, 339-360, 1991xe2x80x9d, the disclosure of which is hereby incorporated by reference. They studied the dissolution of FeOOH (or equivalently, Fe2O3H2O) and found that HEDPA combined with a reducing agent such as sodium formaldehydesulfoxylate (SFS) performed best with the fastest dissolution kinetics.
The advantage of HEDPA is that it is highly effective in dissolving ferrous oxides and retaining the dissolved components in solution. The diphosphonic acids, in general, display very strong chelating ability (high stability) for the trivalent transition metals and higher valency rare earths. Minerals such as magnetite, hematite, ferrite, and other iron-rich spinel phases, can be dissolved while the base-metal substrate is apparently unaffected. Furthermore, due to the thermal instability of diphosphonic acid (DPA), its decomposition produces innocuous speciesxe2x80x94a metal phosphate phase, CO2, and H2O. Similarly, SFS decomposes to SO2 and H2O.
Horwitz et al. did not address the problem of disposing of the material resulting from the treatment with a combination of HEDPA and SFS, but disposal is a significant environmental issue. We have found a method for completing the processing so the process products, other than the phosphate precipitate can be disposed of like other commercial wastes without endangering the environment.